The present invention relates to captive panel fasteners of the type having a retractable screw. The screw is captive and extendible within a ferrule which snaps into one of two panels joined by the fastener.
Snap-in panel fasteners are well known in the art and typically include a screw which is held captive but is free to move axially within a ferrule which is secured to a first panel. The ferrule is affixed to the panel by inserting its shank portion into a punched or drilled hole whereupon resilient metallic fingers collapse until they are free to expand on the reverse side of the panel. The resilient fingers have a groove which snaps into the panel mounting hole as the fingers return to their normal state. The ferrule and thus the entire assembly is held captive in the panel.
The prior art fasteners of the above-described type are typically high in cost as both the screw and ferrule are machined or formed from a metallic bar or wire at a fairly slow production rate. Also, they are available in limited configurations due to the manufacturing processes, and only in a very limited range of colors. Most importantly, prior art snap-in panel fasteners require a secondary operation, and in some cases, the introduction of a third washer-like member in order to captivate the screw in the ferrule. In some designs, should screw threads become damaged, screw removal and replacement is impossible due to the method by which the screw is captivated in the ferrule. Thus, repairs can only be achieved by replacing the entire assembly, increasing repair difficulty and warranty costs.
The users of panel fasteners are also moving away from the traditional configuration of such hardware, and lower cost aesthetic units with color coded, and color-matched screws and ferrules are needed. Screw heads also must meet ergonomic and aesthetic needs and such heads must easily be modified for different drive configurations. In addition, a simpler method of captivating the screw in the ferrule is also needed. Therefore, there is a need in the art for a new type of snap-in panel fastener which meets these requirements.
The present invention meets the needs in the art described above and provides a marked improvement over the prior art by replacing the traditional metallic screw and ferrule with a hybrid screw/plastic cap and a plastic ferrule, thus providing the user with unique advantages.
The screw portion is comprised of a metallic screw insert with an overmolded plastic cap. The screw insert is preferably manufactured by means of a cold forming process which allows fabrication of internal drive recesses, and dog point ends for easy location of mating threads while providing increasing thread strength when compared to conventional cut threads. The screw insert is fabricated with a non-round head which permits the molded plastic knob to transmit tightening torque to the screw insert as well as resistance to axial loads between the knob and the screw inert. An unthreaded neck is sized to allow radial float between the screw axis and the ferrule axis. The knob has large flutes on its outer diameter for ergonomic finger operation. In addition, the knob may have a slot molded in its top to allow screwdriver or coin operation. The knob also has a central cavity that allows a drive tool direct access to the internal drive recess of the metallic screw inert.
In production, screw inserts are loaded into suitable mold cavities in an injection molding press and as molten plastic is injected into the mold cavities, a cap or knob is formed directly on the screw inert. Both the screw knob and the ferrule are molded from an engineered plastic such acetyl copolymer, one example of which is sold under the brand name xe2x80x9cCelconxe2x80x9d(copyright). Other engineered plastics may be used to achieve required flammability ratings. Metallic ferrules are machined from bar stock on a screw machine at a typical rate of 250 pieces per hour. Molten plastic is then injected into suitable mold cavities to produce the required shape. Production rates for injection molded ferrules is typically 5400 pieces per hour using 32-cavity tooling, resulting in substantial cos savings. Additional mold cavities can increase this production rate even higher.
Assembly of the screw to the ferrule is achieved by merely pressing the screw threads through the ferrule body. An integral internal annular retaining ring in the ferrule is dimensioned to be less than the major diameter of the screw thread and equal to the minor diameter. The ring is very narrow such that it allows the screw to be easily pressed through the annular opening, which elastically expands as the threaded portion of the screw passes through it, but then contracts to its normal diameter once the threaded portion has passed. The screw is thus captivated in the ferrule, resisting unintentional disengagement due to vibration or axial loading. However, a damaged or defective screw may still be removed from the ferrule, by applying a light axial force to the screw in a direction away from the panel, and simultaneously applying a counter-clockwise rotation to the screw head, which xe2x80x9cunthreadsxe2x80x9d the screw through the annular opening without damaging the annular ring. A new screw may then be reinserted in a reverse fashion, by applying an axial force toward the panel and applying a clockwise rotation.
The assembly, including the screw and the ferrule, is snapped into the first panel by means of pressing the shank of the ferrule into its properly sized mounting hole. Resilient fingers on the leading edge of the ferrule have lead-in ramps which permit low installation force in order to minimize wear on ferrule surfaces and eliminate marring of finished, painted panels. The panel mounting hole is sized to interlock with the ferrule undercut allowing the fingers to return to an unstressed condition. The flat upper face of the fingers provides high pullout resistance from the panel.
Therefore, the main object of the invention is to provide the advantages described above. Other objects and advantages will be apparent from the following drawings and the description of the preferred embodiment.